Resist film thickness is an important parameter for a lithography process. The incident exposure light interferes with the light reflected from the substrate, forming an interference pattern. The feature size in the resist changes with the resist film thickness. Therefore, the resist film thickness has to be optimized to realize the best process window. The so-called swing-curve, which measures the variations of feature size vs. resist film thickness, is an effective approach to obtain the optimal resist film thickness.
The conventional swing-curve determination needs a series of wafers coated with resists of different thicknesses. The wafers are exposed with a fixed dose, and the feature sizes are measured after development. In a conventional process, different resist thicknesses are usually realized on different wafers. Each wafer is spin coated separately using different rotation speeds to realize certain thicknesses. There are several problems with this approach.
One problem is that too many wafers are needed for an accurate swing curve. Usually, 20-30 points are needed to plot a swing curve. This means that 20-30 wafers are required for the swing curve determination. Another problem is that process variations from wafer to wafer introduce additional errors into the measurements. Therefore, limiting wafer-introduced error requires that the substrates/wafers be as uniform as possible. Also, variations of process conditions from wafer to wafer have to be kept to a minimum.
In light of problems such as these, there remains a need for improved methods for generating resist swing curves in semiconductor manufacturing.